Complexity of the Mw 6.3 2009 L'Aquila (central Italy) earthquake: 1. Multiple finite-extent source inversion

[1] Strong ground motion recordings of the Mw 6.3 2009 L'Aquila earthquake are analyzed by a newly proposed slip inversion technique. The source model consists of Multiple Finite-Extent (MuFEx) subsources. The slip amplitude, rupture velocity, rake and risetime are assumed constant within each subsource. The size and location of the MuFEx subsources have to be inferred independently from other methods, preferably those free of strong constraints (such as constant rupture velocity over the whole fault, etc); here we use two published approaches satisfying such requirement: the truncated singular value decomposition and the iterative multiple-point source deconvolution. Each MuFEx subsource is characterized by an individual set of trial nucleation points, rupture velocities and nucleation times, which are grid-searched. For each combination of these parameters, the subsources' slip is determined by the least squares approach. This procedure thus provides not only the best fitting model, but also a whole range of acceptable models, allowing for the uncertainty analysis. The method is demonstrated on three synthetic tests. When applied to the L'Aquila data, both the best fitting model and the uncertainty analysis suggest that the event consisted of two major episodes, one with the rupture propagating immediately after the nucleation in the updip direction, the other being delayed by 3–4 s with dominant propagation toward the SE along the deeper part of the fault. The retrieved complexity of the rupture propagation warns against attempts to stabilize inversions by the use of a constant rupture velocity over the whole fault.